1. Field of the Invention
The present invention relates to a variable magnification optical system, and more specifically relates to a compact, high-magnification zoom viewfinder optical system of the real image type which is provided separately from the photographic lens of a lens-shutter type camera or the like.
2. Description of the Related Art
In recent years, the demand has grown for cameras having greater functionality. Demand has also increased for viewfinder units, having correspondingly high variable magnification, for use in cameras which are provided with a viewfinder as a completely separate unit from the photographic system. However, in the case of zoom viewfinder systems, it becomes difficult to assure that the viewfinder will have suitable aberration characteristics over the entire zoom range in conjunction with the high variable magnification.
Examples of conventional systems utilizing a variable magnification ratio in the range of about 3 to about 4 are disclosed in Japanese Laid-Open Patent Application Nos. 2-173713, 2-173714, 2-191908, and 6-102453. A characteristic of these viewfinder systems is the provision of a positive/negative/positive objective optical system.
The conventional positive/negative/positive construction readily assures suitable aberration characteristics over the entire zoom range of a magnification ratio less than about 4, and is particularly suitable for high variable magnification in the range of about 3 to about 4. However, at a high variable magnification ratio in the range of about 5 to about 6, it is rather difficult to assure adequate aberration characteristics over the entire zoom range.
In cameras provided with a viewfinder as a separate unit completely independent of the photographic system, the viewfinder unit must satisfy the reciprocal conditions of compactness and high variable magnification. Generally, when the aspect of compactness in the overall length direction is satisfied, the aspect of high variable magnification is not satisfied; and when the aspect of high variable magnification is satisfied, the aspect of compactness is not satisfied. The length of the viewfinder objective system is important in reducing the thickness of the system in the overall length direction.
In the aforesaid conventional constructions having a variable magnification in the range of about 3 to about 4 and being provided with a positive/negative/positive objective optical system, there is a loss of compactness due to an increase in the overall length of the objective system when obtaining high variable magnification.
An object of the present invention is to provide a variable magnification optical system which satisfies at a high level the demands of reciprocity of high variable magnification and compactness, even at a variable magnification ratio of 5 or greater.
Another object of the present invention is to provide a variable magnification viewfinder which assures suitable aberration characteristics over the entire zoom range, even at a high magnification ratio of 5 or greater.
In accordance with a first aspect of the invention, these objects are attained by a zoom optical system, having a wide angle position and a telephoto position, which is provided with an objective optical system having a four lens unit construction comprising, sequentially from the object side: a first lens unit having a positive optical power, a second lens unit having a negative optical power, a third lens unit having a positive optical power, and a fourth lens unit having either a negative optical power or a positive optical power; wherein each of said second lens unit and said third lens unit is movable in a direction along the optical axis; and wherein the following conditional relationship is satisfied:
0.15xe2x89xa6xcex24W/FLWobjxe2x89xa60.28
where:
xcex24W represents the image forming magnification of the fourth lens unit at the wide angle end, and
FLWobj represents the focal length of the objective optical system at the wide angle end.
In accordance with a second aspect of the invention, these objects are attained by a zoom optical system, having a wide angle position and a telephoto position, which is provided with an objective optical system having at least a three lens unit construction comprising, sequentially from the object side: a first lens unit having a positive optical power, a second lens unit having a negative optical power, and a third lens unit having a positive optical power; and wherein each of said second lens unit and said third lens unit is movable in a direction along the optical axis; and wherein said third lens unit comprises a single optical power element as the only optical power element in the third lens unit, said single optical power element being a biconvex lens element which satisfies the following conditional relationship:
Nd3xe2x89xa71.6
where Nd3 represents the d-line refractive index of the lens material of the biconvex lens element of the third lens unit.
It is presently preferred that the invention also satisfy the following conditional relationship:
0.038xe2x89xa61/FL1xe2x89xa60.068
where:
FL1 represents the focal length of the positive first lens unit.
Also, it is presently preferred that the third lens unit has at least one surface which is an aspherical surface which, relative to a height y in an optional direction perpendicular to the optical axis direction such that 0.7Ymax less than y less than Ymax (wherein Ymax is the maximum height of the aspherical surface in a direction perpendicular to the optical axis), satisfies the following conditional relationship:
xe2x88x920.07 less than xcfx863xc2x7(Nxe2x80x2xe2x88x92N)xc2x7(d/dy) {x(y)xe2x88x92x0(y)} less than 0
where:
xcfx863 represents refracting power of the third lens unit,
N represents d-line refractive index of the medium of said third lens unit for an aspherical surface on the object side,
Nxe2x80x2 represents d-line refractive index of said medium for an aspherical surface on the image side,
x(y) represents a shape of the aspherical surface, and
x0(y) represents a reference spherical surface shape of the aspherical surface.
The values x(y) and x0(y) can be represented as follows:
x(y)=(r/xcex5)xc2x7[1xe2x88x92{1xe2x88x92xcex5xc2x7(y2/r2)}]+xcexa3Aiyi (where ixe2x89xa72),
and
x0(y)=r#xc2x7[1xe2x88x92{1xe2x88x92(y2/r#2)}]
where:
r represents a standard radius of curvature of the aspherical surface,
xcex5 represents a secondary curved surface parameter,
Ai represents a aspherical surface coefficient of an i degree, and
r# represents a paraxial radius of curvature of the aspherical surface (where 1/r#=(1/r)+2xc2x7A2).
Also, in a presently preferred embodiment of the invention, the positive first lens element is a single lens element formed of glass having a low dispersion which satisfies the following conditional relationship:
xcexdd greater than 65
where xcexdd represents d-line Abbe""s number.
The zoom optical system can be a variable magnification viewfinder of the real image type which is separate from the photographic system, wherein variable magnification is accomplished by moving the second lens unit in the optical axis direction, and wherein diopter variation due to variable magnification is corrected by moving at least said third lens unit in the optical axis direction.
These and other objects, advantages and features of the present invention will become apparent from the following descriptions thereof taken in conjunction with the accompanying drawings which illustrate specific examples of the invention.